The present invention relates generally to DC offset compensation, and more particularly, to a gain stage with DC offset compensation.
Communications receivers often suffer from interference due to a variety of factors, one of which is direct current (DC) offset. DC offset is usually undesirable since it may cause saturation or changes in the operating point of an amplifier of the receiver, especially when the amplifier operates with a relatively high gain. Conventionally, a DC offset canceller is one effective way to remove the DC offset interference.
Although the DC offset canceller can remove the DC offset interference under most conditions, the DC offset canceller disadvantageously brings further noise to the amplifier, which is especially unfavorable for noise-sensitive applications, such as an intermediate frequency (IF) gain stage of a frequency modulation (FM) receiver.
A gain stage of an FM receiver is schematically illustrated in FIG. 1. The gain stage 100 comprises a gain amplifier 110 and a DC offset canceller 120. The gain amplifier 110 varies its gain based on a gain control signal S_Gain (which is used to adjust variable components to vary the gain), and accordingly amplifies an input IN of the gain amplifier 110. The DC offset canceller 120 detects DC offset at the output OUT of the gain amplifier 110 and accordingly generates a compensation signal S_C which is fed back to the input IN of the gain amplifier 110 to cancel the DC offset. Since the DC offset canceller 120 needs to cover any possible DC offset, it is necessary to design the DC offset canceller 120 to have capability of providing a compensation amount whose maximum value is sufficient for any possible DC offset. As mentioned above, however, the DC offset canceller 120 disadvantageously brings further noise to the gain amplifier 110. If the DC offset canceller 120 is designed to provide a compensation amount having a larger maximum value than necessary (e.g. when the gain amplifier 110 is operated with a relatively low gain, the DC offset will be relatively low), the DC offset canceller 120 will unnecessarily interfere with the gain amplifier 110 since the cancellation of the DC offset does not need such a large compensation amount. This means the DC offset canceller 120 will affect the gain amplifier 110 with unnecessary noise.
Accordingly, there is a need in the art for a DC offset canceller that can alleviate noise caused to the amplifier.